This is a proposal for five years of continued support for the National Biomedical ESR Center, which was established in May 1976 under the sponsorship of Biotechnology Resources Branch of NIH. The facility occupies 8,000 sq. ft. in a separate building, is staffed by 9 ESR spectroscopists, and has 4 major ESR spectrometer stations with ENDOR, ELDOR, Q-band, Light Modulation Spectroscopy, and Saturation Transfer capability. Pulse ESR Instrumentation is under construction. It is probably the largest biomedically-oriented ESR facility in the world. Collaborations have been extensive-- with 34 projects in Year 2 involving scientists from 15 other institutions. Center personnel authored 26 papers during Years 1 and 2, mostly on a collaborative basis. A program to facilitate use of the Center by promising young investigators has been developed. The primary Service function has been to provide advice and consultatiion on ERS Spectroscopy. More than 100 such contacts have been logged and there were 69 professional visitors to the Center during years 1 and 2. In addition, instruments are made available to visitors and sample sent to us are run. The Center will have an increasing role in facilitating communication in ESR Spectroscopy via newsletters and by acting as a referral center. Education and Training has involved courses, outside lectures, seminars, and on-site training of medical students, graduate students, visitors, and postdoctoral fellows. Workshops in specialized and general aspects of ESR Spectroscopy are planned. Core R & D is intended to provide leadership in development of ESR spectrometer systems. Prototype biomedical applications of the new instruments are carried out. An interlocking program of development of ESR-Computer technology and Pulse ESR instrumentation is proposed which includes not only apparatus for new types of pulse experiments but also construction of the Next Generation of General Purpose ESR Spectrometer Systems. Another prominent area of core R & D is construction of Microwave Bridges operating from 2 to 35 GHz.